1. Field of the Invention
This invention relates to a system for controlling the locomotion of a legged walking robot and, more particularly, to a system for controlling the locomotion of a biped walking robot or the like which prevents lag from arising in joint drive control.
2. Description of the Prior Art
A legged walking robot, more specifically a biped walking robot, is disclosed in Japanese Laid-open Patent Publication No. 62-97006. As shown in FIG. 11, the forces acting on the center of gravity G of the robot during walking are a vertical force owing to the robot's weight and an inertial force resulting from acceleration associated with walking, and the resultant of these two forces has to be balanced by an equal an opposite force from the ground. Unless the positioning and timing of foot placement are in accordance with the prescribed walking pattern, this balance cannot be ensured and the robot's locomotion is apt to become unstable.
The conventional way of achieving joint angles faithful to the command values in a legged walking robot has been to use what is known as feedback control. In this type of control, after the angle command value has been applied to the joint drive motor, the resulting actual joint angle is measured by an encoder or the like attached to the motor, whereafter the actual and command values are compared to determine the control deviation and the joint drive motor is operated by an amount to minimize the control deviation. However, for theoretical reasons that explained below, such feedback control inevitably involves some degree of deviation and, moreover, the amount of the deviation differs from one joint to another. The resulting control deviations or lags destabilizes the robot's gait, disturbs the locus of the center of gravity and upsets the locomotion timing, so that the force received from the ground also tends to deviate from the proper magnitude and direction. Stable locomotion thus becomes difficult or impossible. The reason why the deviation leads to control lag will be understood from FIG. 12, which is a block diagram showing the general principle of feedback control. Assuming that the angular velocity command value applied to the motor is constant, the deviation between the encoder detection value .theta.R and the command value .theta.COMM is multiplied by an appropriate gain kp and the product is input to the motor as a velocity command value omega COMM, causing the motor to rotate by an amount omega R equal to omega COMM. However, owing to the system equilibrium EQU .theta.COMM-.theta.R)kp-omega R=0
Therefore EQU .theta.COMM-.theta.R=omega R/kp.noteq.0
and from this it can be seen that a deviation proportional to omega R occurs. FIGS. 13 and 14 show data measured during actual use of this conventional control, FIG. 13 showing the relationship between the command values and the actual joint angles and FIG. 14 showing the corresponding lag. As can be seen from these figures, lag arises when the conventional control is employed.
While one conceivable way of reducing this control deviation is by increasing the feedback gain, this raises the servo rigidity to a higher level than required and is likely to give rise to oscillation. Moreover, since it is necessary from the viewpoint of the practicality of a legged walking robot to take weight, price and power consumption factors into consideration, there is a limit on the performance of the computer that can be installed in the robot. This makes it necessary to solve the problems related to the highly complex control required for locomotion of a walking legged robot by use a relatively low-performance computer. For this, the logic of the motor feedback control has to be simplified as much as possible in order to reduce the load on the computer.
One object of the invention is therefore to provide a system for controlling the locomotion of a legged walking robot which is of simple configuration but is nevertheless able to prevent control deviation and ensure stable walking.
The earlier mentioned Japanese Laid-open Patent Publication No. 62-97006 discloses a control method with excellent possibility of enabling realization of a practicable robot. In this method, the joint angles are calculated offline as a time series in advance and the calculated data is output by an on-board computer during locomotion. Since according to this method data calculated by a computer independent of the robot is stored in the memory of the robot's on-board computer and used during locomotion to control the walking, the on-board computer can be a relatively low-performance machine.
The second object of the invention is therefore to provide a system for controlling the locomotion of a legged walking robot which, in a control method using time series locomotion data calculated offline, is able to prevent control deviation and ensure stable walking in a simpler manner than has been possible heretofore.